Generally, a surge tank is made of a metallic material such as aluminum or the like. One end of the surge tank integrally couples to a throttle body.
The surge tank is fixed to the engine via a protrusive flange and a stay. The protrusive flange is formed at one end of the surge tank, and the stay connects the protrusive flange and the engine therebetween.
In case a plastic surge tank is applied for reducing the engine weight, the throttle body directly coupled and supported by the surge tank is made primarily of a metal, e.g., aluminum. As a result, when the plastic surge tank and metallic throttle body are coupled to each other, the balance of the weight is concentrated on the heavy throttle body.
Furthermore, the firm supporting force conventionally provided from the metallic surge tank to the throttle body cannot be expected from the surge tank made of a plastic material.